1. Field of the Invention
The invention relates to improvements in a cooking stove having a smooth-top glass ceramic cooktop, and a smooth-top glass ceramic cooktop with a glass ceramic cooktop cooking surface, to a method for production of stoves with smooth-top glass ceramic cooktops and smooth-top glass ceramic cooktops.
2. Background Information
Smooth-top glass ceramic cooktop cooking surfaces of smooth-top kitchen ceramic or glass ceramic cooktops or a stove having a ceramic or glass ceramic cooktop cooking surface have gained considerable popularity as kitchen appliances.
Thus, cooking appliances having ceramic or glass ceramic cooking surfaces are known.
They provide a substantially smooth upper surface on which the various utensils that are to be heated can be disposed.
In these appliances, the cooking zones can be heated, as a rule, by means of electrically operated or gas operated heating devices arranged below the ceramic or glass ceramic cooking surface in the region of the cooking zones. These devices can be, for example, electrically operated contact-heating or radiant heating elements or else gas-jet burners.
An example of a cooktop comprises an induction heating apparatus having a cooktop including a plurality of induction surface heating units. The cooktop comprises a horizontally disposed planar metal support surface having a plurality of openings therein. A ceramic smooth-top plate is supported in each of the openings and adapted to support a cooking utensil thereon. An induction heating coil is supported subjacent to the ceramic plate in a position to generate a magnetic field which passes through the plate to link the cooking utensil. Each plate is supported by a metallic trim frame, which abuts a conductive layer on the plate, with the frame and layer combining to provide a low reluctance flux path, the low reluctance path operating to reduce the magnetic flux leaked into the space surrounding the heating apparatus during operation thereof.
Another example of a cooktop has a heating unit that includes two tubular tungsten-halogen lamps, each having a tungsten filament. The lamps are supported within a ring of ceramic fibre material and the unit is preferably mounted beneath an infra-red-transmissive cooktop to define a hotplate area of a cooking hob. A control circuit provides a range of discrete power outputs of the lamps, each power output corresponding to a power control setting set by a user of the cooking hob. The circuit includes a phase control circuit for switching power to the lamps at a predetermined phase angle to achieve one or more of the lower power outputs.
Yet another example of a cooktop comprises a burner for a xe2x80x9csealed topxe2x80x9d range which has a generally upwardly diverging conical body with radially disposed fuel ports and a generally flat removable cap disposed on the upper periphery of the body this invention is a translucent glass ceramic, a method for its production and its use.
Furthermore, it is known that glass made from the system lithium oxide-aluminium oxide-silicon dioxide can be transformed into glass ceramics (LAS glass ceramics) with high quartz mixed crystals and/or keatite mixed crystals as the main crystal phases. These glass ceramics are manufactured in a number of different stages. After the fusion and hot forming, the material is conventionally cooled to below the inversion temperature. In other words, the material may be cooled to below the transformation temperature. The initial glass is then transformed by controlled crystallization into a glass ceramic item. This ceramization takes place in a multiple-stage temperature process, in which first by nucleation at a temperature between approximately six hundred degrees Celsius to eight hundred degrees Celsius nuclei or seeds, generally consisting of titanium dioxide or zirconium dioxide/titanium dioxide mixed crystals, are generated, although tin dioxide can also participate in the nucleation. During the subsequent temperature increase, at the crystallization temperature of approximately seven hundred and fifty degrees Celsius to nine hundred degrees Celsius, first high quartz mixed crystals form on these nuclei. As the temperature is increased further in the range of approximately nine hundred degrees Celsius to twelve hundred degrees Celsius, these high quartz mixed crystals are further transformed into keatite mixed crystals. The transformation into keatite mixed crystals is accompanied by a crystal growth, i.e. increasing crystallite size, as a result of which there is an increasing diffraction of light, i.e. the light transmission becomes less and less. The glass ceramic item thereby appears increasingly translucent and finally opaque. The glass ceramics with high quartz mixed crystals are usually transparent, and translucent glass ceramics can also be manufactured by reducing the concentration of nucleation agents.
A key characteristic of these glass ceramics is that they are manufactured with materials that have an extremely low coefficient of thermal expansion in the range from room temperature up to approximately seven hundred degrees Celsius of less than one and five tenths millionths per degree Kelvin. With glass ceramics that contain high quartz mixed crystals as the main crystal phase, even materials with almost zero expansion can be realized in a specified temperature range, e.g. between room temperature and seven hundred degrees Celsius.
These glass ceramics are used in transparent form, for example, as fire protection glass, smokestack view windows or cookware. For use as a cooking surface, it is desirable to reduce the light transmission, to make it impossible to see through the surface to the equipment installed underneath. This reduction of light transmission can be achieved, for example, by coloring transparent glass ceramics as well as by using translucent or opaque transformed glass ceramics.
For example, WO 99/06334 describes a translucent glass ceramic of the prior art which has a degree of opacity of at least fifty-percent. WO 99/06334 also claims a corresponding translucent glass ceramic with a transmission in the visible range of five percent to forty percent. The above mentioned translucent glass ceramics thereby contain either beta-spodumene (keatite mixed crystals) as the predominant crystal phase or exclusively beta-spodumene as the only crystal phase.
European Patent 0 437 228 A1 (corresponding to U.S. Pat. No. 5,070,045 issued to Comte et al. on Dec. 3, 1991 and entitled, xe2x80x9cTransparent glass-ceramic article,xe2x80x9d) describes a transparent glass ceramic with beta-quartz mixed crystals (high quartz mixed crystals) as the predominant crystal phase or a white opaque glass ceramic with beta-spodumene mixed crystals (keatite mixed crystals) as the predominant crystal phase.
The variable-translucence glass ceramic described in European Patent 536 478 A1 (corresponding to U.S. Pat. No. 5,173,453 issued to Beall et al. on Dec. 22, 1992 and entitled, xe2x80x9cVariably translucent glass-ceramic article and method for making,xe2x80x9d) contains, in addition to areas with beta-quartz mixed crystals, areas with beta-spodumene/gahnite mixed crystals. These gahnite mixed crystals (zinc oxide-aluminum oxide) are formed during the phase transformation of beta-quartz mixed crystals into beta-spodumene mixed crystals and compensate for the change in density that accompanies this phase transformation. The immediate vicinity of transparent, translucent and opaque areas can therefore be transformed in a glass ceramic item. In the translucent and opaque areas, keatite mixed crystals are the main crystal phase. Gahnite crystals have a significantly higher coefficient of thermal expansion than the above mentioned mixed crystal phases (high quartz or keatite) of typical LAS glass ceramics. It can be expected that a variably crystallized product of this type has disadvantages in its impact strength and will develop structural cracks fairly early in actual use on account of the different expansion characteristics.
U.S. Pat. No. 4,211,820 issued to Cantaloupe et al. on Jul. 8, 1980 and entitled, xe2x80x9cBrown glass-ceramic articles,xe2x80x9d describes essentially transparent glass ceramics with slight opacity, with beta-spodumene mixed crystals as the predominant crystal phase in the interior of the glass ceramic. By means of two hundredths weight percent to two tenths weight percent of vanadium pentoxide, the transparent glass ceramics claimed there are tinted brown. A comparable glass ceramic is described in U.S. Pat. No. 4,218,512 issued to Allersma on August 1980 and entitled, xe2x80x9cStrengthened translucent glass-ceramics and method of makingxe2x80x9d.
One object of the invention is to provide a smooth smooth-top cooktop glass ceramic comprising at least one of: (a) an upper layer of said glass ceramic being configured to minimize surface defects; said surface defects comprising at least one of: fissures, cracks, pits, and pores; (b) an inner layer of said glass ceramic being configured to provide resistance to impact to said upper layer from cooking utensils being dropped onto said upper layer of said smooth-top cooktop; and (c) at least said inner layer of said glass ceramic being configured to obscure visibility, through said upper layer of said glass ceramic, of said at least one heat source.
Another object of the invention is to find a translucent glass ceramic and a method for the manufacture of a translucent glass ceramic, whereby the glass ceramic has a light transmission in the visible range of five tenths percent to ten percent at a specimen thickness of four millimeters, an impact strength of more than eighteen centimeters drop ball impact strength on average, tested with a two hundred grams steel ball in the ball drop impact strength test, and can withstand high temperature differences of more than six hundred and fifty degrees Celsius.
In accordance with one aspect the invention teaches that these and other objects can be accomplished by a stove, with a smooth-top cooktop, for cooking food, said stove comprising: a stove body; said stove body being configured with a support comprising feet; a smooth-top cooktop to cook food thereon; an arrangement to attach said smooth-top cooktop to said stove body; said smooth-top cooktop comprising a layer which becomes an upper layer upon installation in a kitchen; said smooth-top cooktop comprising a cooking surface configured to cook food; said cooking surface being disposed at or adjacent to said upper layer of said smooth-top cooktop; said cooking surface, at or adjacent to said upper layer of said smooth-top cooktop, being configured to receive a bottom of a cooking vessel to cook food therein; at least one heat source; said at least one heat source being configured to heat said cooking surface to thereby cook food; said at least one heat source being disposed adjacent to said cooking surface; control apparatus being configured and disposed to control said smooth-top cooktop; said control apparatus being connected to control said at least one heat source and thus to control heat; said smooth-top cooktop comprising glass ceramic; said cooking surface being disposed adjacent to or comprising a part of said glass ceramic; said glass ceramic comprising said upper layer and an inner layer contiguous to said upper layer; said inner layer of said glass ceramic being disposed below said upper layer of said glass ceramic upon installation of said stove in a kitchen; said upper layer and said contiguous inner layer of said glass ceramic comprising a glass ceramic structure being continuously glass ceramic from said upper layer through said inner layer; said upper layer comprising a first glass ceramic material; said inner layer comprising a second glass ceramic material; said first glass ceramic material comprising a different glass ceramic material than said second glass ceramic material; said upper layer of said glass ceramic being configured to minimize surface defects; said surface defects comprising at least one of: fissures, cracks, pits, and pores; said inner layer of said glass ceramic being configured to provide resistance to impact to said upper layer from cooking utensils being dropped onto said upper layer of said smooth-top cooktop; and at least said inner layer of said glass ceramic being configured to obscure visibility, through said upper layer of said glass ceramic, of said at least one heat source.
The invention also teaches that the object can be accomplished by a glass ceramic comprising a light transmission or, in other words, a light transmissivity, in the visible range of five tenths percent to ten percent with four millimeters specimen thickness, an impact strength of more than eighteen centimeters drop ball impact strength on average, tested with a two hundred grams steel ball in the ball drop test, an ability to withstand temperature differences of more than six hundred and fifty degrees Celsius, keatite mixed crystals as the predominant crystal phase in the surface layer of the glass ceramic, high quartz mixed crystals as an additional crystal phase in the surface layer of the glass ceramic, a coefficient of thermal expansion of the high quartz mixed crystals which is less than that of the keatite mixed crystals, so that a surface condition of the glass-ceramic is achieved that counteracts the origin of superficial injuries that reduce strength, and a silicon dioxide content of the high quartz mixed crystals that is less than eighty weight percent, so that as the glass ceramic cools to room temperature, the transformation of the high quartz mixed crystals phase into an undesirable low quartz mixed crystals phase, which leads to cracks in the surface of the glass ceramic, is prevented.
The invention also teaches a method for the production of a translucent, essentially plate-shaped glass ceramic with a light transmission in the visible range of five tenths percent to ten percent with four millimeters specimen thickness, an impact strength of more than eighteen centimeters drop ball impact strength on average, tested with a two hundred grams steel ball in the ball drop test, an ability to withstand temperature differences of more than six hundred and fifty degrees Celsius, keatite mixed crystals as the predominant crystal phase in the surface layer of the glass ceramic, high quartz mixed crystals as an additional crystal phase in the surface layer of the glass ceramic, a coefficient of thermal expansion of the high quartz mixed crystals which is less than that of the keatite mixed crystals, so that a surface condition of the glass ceramic is achieved that counteracts the origin of superficial injuries that reduce strength, a silicon dioxide content of the high quartz mixed crystals that is less than eighty weight percent, so that as the glass ceramic cools to room temperature, the transformation of the high quartz mixed crystals phase into an undesirable low quartz mixed crystals phase, which leads to cracks in the surface of the glass ceramic, is prevented, whereby the laminated structure with keatite mixed crystals as the predominant crystal phase in the interior of the glass ceramic and high quartz mixed crystals as the predominant crystal phase in the surface layer of the glass ceramic is produced by the fact that the temperature range of the nucleation of nucleus crystals containing zirconium/titanium from six hundred and fifty degrees Celsius to seven hundred and sixty degrees Celsius is traversed at high heating rates of more than seven degrees Kelvin per minute, the crystallization of the high quartz mixed crystal phase is performed at a temperature from seven hundred and sixty degrees Celsius to eight hundred and fifty degrees Celsius, and the hold time in the temperature range between six hundred and fifty degrees Celsius to eight hundred and fifty degrees Celsius is less than sixty minutes.
The translucent glass ceramic taught by the invention thereby has:
a light transmission in the visible range from five tenths percent to ten percent at a specimen thickness of four millimeters,
an impact strength of more than eighteen centimeters drop ball impact strength on average, tested with a two hundred grams steel ball in the falling ball impact test,
the ability to withstand a temperature difference of more than six hundred and fifty degrees Celsius, preferably greater than seven hundred degrees Celsius,
keatite mixed crystals as the predominant crystal phase in the interior of the glass ceramic,
high quartz mixed crystals as an additional crystal phase in the surface layer of the glass ceramic,
a coefficient of thermal expansion of the high quartz mixed crystals that is less than that of the keatite mixed crystals, so that a surface condition of the glass ceramic is produced that counteracts the origin of surface damage that reduces the strength of the glass ceramic, and
a silicon dioxide content of the high quartz mixed crystals that is less than eighty weight percent, so that as the glass ceramic cools to room temperature, the transformation of the high quartz mixed crystal phase to an undesirable low quartz mixed crystal phase, which leads to cracks in the surface of the glass ceramic, is prevented.
In a method taught by the invention for the manufacture of a translucent glass ceramic with a light transmission or, in other words, light transmissivity, in the visible range from five tenths percent to ten percent at a specimen thickness of four millimeters, an impact strength of more than eighteen centimeters cm drop ball impact strength on average tested with a two hundred grams steel ball in the ball drop impact strength test, the ability to withstand a temperature difference of more than six hundred and fifty degrees Celsius, preferably greater than seven hundred degrees Celsius, with keatite mixed crystals as the predominant crystal phase in the interior of the glass ceramic, with high quartz mixed crystals as the crystal phase in the surface layer of the glass ceramic, a coefficient of thermal expansion of the high quartz mixed crystals that is less than that of the keatite mixed crystals, so that a surface condition of the glass ceramic is produced which counteracts the origin of surface damage that reduces the strength of the glass ceramic, a silicon dioxide content of the high quartz mixed crystals which is greater than eighty weight percent, so that as the glass ceramic cools to room temperature, the transformation of the high quartz mixed crystal phase to an undesirable low quartz mixed crystal phase, which leads to cracks in the surface of the glass ceramic, is prevented, the lamination with keatite mixed crystals as the predominant crystal phase in the interior of the glass ceramic and high quartz mixed crystals as an additional crystal phase in the surface layer of the glass ceramic, because the temperature range of six hundred and fifty degrees Celsius to seven hundred and sixty degrees Celsius of the nucleation of seed crystals or crystal nuclei that contain zirconium/titanium is traversed at high heating rates of more than seven degrees Kelvin per minute, the crystallization of the high quartz mixed crystal phase is performed at a temperature from seven hundred and sixty degrees Celsius to eight hundred and fifty degrees Celsius, and the hold time in the temperature range between six hundred and fifty degrees Celsius and eight hundred and fifty degrees Celsius is less than sixty minutes.
During the crystallization of the high quartz mixed crystal phase in the temperature range from seven hundred and sixty degrees Celsius to eight hundred and fifty degrees Celsius, the glass ceramic can be held at a defined temperature, although it is also possible to traverse at least something of a temperature range. In other words, the mentioned temperature range can be traversed at least in part.
The glass ceramic taught by the invention or the glass ceramic manufactured by the method taught by the invention preferably has high quartz mixed crystals as the predominant crystal phase in the surface layer of the glass ceramic.
The ability of the glass ceramic to withstand temperature differences is indicated by the following expression: delta T is equal to the result of the division comprising the numerator of sigma g times one minus mu and the denominator alpha times E, where delta T is the ability to withstand temperature differences, mu is the Poisson ratio, E is the modulus of elasticity, alpha is the coefficient of thermal expansion and sigma g is the strength that must be used for the value that is set in practical use as a result of surface damage.
Because the thermal expansion of the high quartz mixed crystals is less than that of the keatite mixed crystals, so that a surface condition of the glass ceramic is achieved that counteracts the origin of surface injuries that would reduce the strength of the glass ceramic, there is reason to suspect that a compression stress is generated on the surface of the glass ceramic that counteracts both external tensile stresses caused by impact loads as well as the generation of surface damage that would reduce the strength of the glass ceramic in practical use. As a result, the level of strength after the type of damage suffered in normal use is higher. This effect is also known with compression stresses that are produced by chemical prestressing, e.g. by the replacement of potassium with sodium in glass. Chemical prestressing, however, cannot be used on cooking surfaces, because the compression stresses are then reduced again by the high temperatures in the cooking zones.
To achieve the ability to withstand high temperature differences, essentially the strength after the sort of damage suffered in actual practice sigma g should be high and the coefficient of thermal expansion alpha should be low. The modulus of elasticity and the Poisson ratio can be influenced only to a small extent by the composition and the manufacturing process. For example, it is advantageous if the thermal expansion of the glass ceramic between room temperature and seven hundred degrees Celsius is less than one and three tenths millionths per degree Kelvin, and preferably less than one and one tenth millionths per degree Kelvin.
Simulation calculations of a plate-shaped cooking surface made of the translucent glass ceramic claimed by the invention using finite element methods show that when the cooking zones are operated correctly, tangential tensile stresses occur on the outer edge of the plate. With the transformation method taught by the invention, a surface condition is produced on the outer edge of the plate which, even after damage suffered during normal wear, has a high strength sigma g. That means that the ability to withstand temperature differences is sufficiently high for the glass ceramic to be used as a cooking surface.
To protect the glass ceramic from the effects of acid on the high quartz mixed crystals, it is advantageous if a vitreous layer approximately one hundred nanometers to six hundred nanometers thick is produced on the immediate surface above the high quartz mixed crystals. In this vitreous layer, the components that are not incorporated into the high quartz mixed crystals, e.g. alkali oxides such as sodium oxide, potassium oxide and earth alkali oxides such as calcium oxide, strontium oxide and barium oxide are enriched. To produce the protective vitreous layer, therefore the above mentioned alkali and earth alkali oxides should be ingredients in the composition. It is known that high quartz mixed crystals can be destroyed by the effects of acid, as a result of which the lithium is replaced by protons from the acid. With the destruction of the high quartz mixed crystals in the layer close to the surface, their positive influence on the ability to withstand temperature differences is also lost.
The laminated structure described above with a vitreous layer that is approximately one hundred nanometers to six hundred nanometers thick, a layer near the surface with high quartz mixed crystals and the keatite mixed crystals in the interior of the glass ceramic can be produced during the ceramization of the glass ceramic by running through the temperature range of the nucleation of seed crystals containing zirconium/titanium from six hundred and fifty degrees Celsius to seven hundred and sixty degrees Celsius at high heating rates of more than seven degrees Kelvin per minute, and the crystallization of the high quartz mixed crystal phase is performed at a temperature of approximately seven hundred and sixty degrees Celsius to eight hundred and fifty degrees Celsius. On plate-shaped glass, it has been found to be advantageous to avoid uneven spots caused by uneven crystallization in the plate if the heating rate is slowed down between seven hundred and sixty degrees Celsius to eight hundred and fifty degrees Celsius or a hold time, or, in other words, a residence time, is introduced. Overall, the hold time in the temperature range between six hundred and fifty degrees Celsius to eight hundred and fifty degrees Celsius should be less than sixty minutes, otherwise a worsening of the ability to withstand temperature differences is observed.
The temperature maximum of the manufacturing process is at temperatures from one thousand degrees Celsius to eleven hundred degrees Celsius. This is where the transformation into the translucent glass ceramic taught by the invention with a light transmission of five tenths percent to ten percent with a four millimeters specimen thickness occurs. The heating-rates and the hold time at the maximum temperature must be selected so that on one hand the desired translucence is achieved, i.e. light transmission of five tenths percent to ten percent at four millimeters specimen thickness. On the other hand, the hold time should not be extended to the point where the high quartz mixed crystal phase has undesirably high silicon dioxide levels in the surface. Silicon dioxide levels on the order of magnitude of greater than eighty percent can result in a transformation from the high quartz structure to a low quartz structure during cooling. It has been observed that the composition of the high quartz mixed crystals varies with the hold time. As the hold time increases, the high quartz mixed crystal phase becomes increasingly richer in silicon dioxide, so that the hold time at the maximum temperature must be limited by process control measures. The allowable hold time depends on the composition of the glass ceramic, the heating rate and the level of the maximum temperature.
Thin-film X-ray diffraction is used to determine the allowable hold, or, in other words, residence, time for the specific process with a given maximum temperature and heating rate. The crystal quartz phase in the surface is measured by measuring with a grazing incidence angle of approximately six tenths degrees. The principal reflections of the crystal quartz phase are at two-theta values of twenty and five tenths degrees and twenty-six degrees. From the diffraction diagrams, the a-lattice constant of the respective crystal quartz phase can be determined. The a-lattice constant of the elementary cell reflects the silicon dioxide content of the quartz phase. Low a-lattice constants stand for high silicon dioxide contents. For each glass ceramic composition, a limit value for the a-lattice constant can be determined at which, on account of high silicon dioxide contents of greater than eighty percent in the quartz phase during cooling, the undesired transformation into the low quartz structure occurs. The limit values for the a-lattice constant are on the order of magnitude of five angstroms to five and four hundredths angstroms.
The transformation of the high quartz mixed crystals into the low quartz structure must be avoided at all costs, because it results in a volume contraction, and on account of the high coefficient of thermal expansion of low quartz compared with the keatite mixed crystal phase in the interior of the glass ceramic, it leads to high stresses in the surface. As a result of the high tensile stresses, surface cracks appear in the glass ceramic item which drastically reduce the impact strength below the required level. The prevention of surface cracks is decisive to achieve the required high impact strengths of eighteen centimeters on average, tested with a two hundred grams steel ball in the drop ball impact test based on DIN 52306. The transformation of the high quartz mixed crystals into the low quartz structure can be reliably determined by the microscopically detectable surface cracks and the related reduction in impact strength. Using this method it is possible to empirically determine the allowable hold time at the specified maximum temperature and heating rate. FIG. 4 shows, for a given composition (Glass No. 1 in Table 1), how the surface cracks occur as a consequence of excessive hold times at a specified heating rate. For each glass ceramic composition, the respective limit value for the a-lattice constant measured with thin-film X-ray diffraction can also be determined. This procedure is illustrated in FIG. 5 for the probes that are identical to those illustrated in FIG. 4. In this example, the critical silicon dioxide content for the undesirable transformation of the high quartz mixed crystals into the low quartz structure lies at values of the lattice constant of a less than approximately five and two hundredths angstroms.
To achieve a the ability to withstand large temperature differences, it has been found to be advantageous if the average grain size of the keatite mixed crystals in the interior of the glass ceramic is three tenths micrometers to two micrometers and preferably one micrometer to one and five tenths micrometers. The upper limit results from the fact that with larger average grain sizes, i.e. a coarse structure, undesirably high microstresses result. The average grain size should be not less than three tenths micrometers and preferably not less than one micrometer, because otherwise the propagation of cracks geometrically is not sufficiently prevented. In the range of medium grain sizes of three tenths micrometers to two micrometers, a high strength with normal practical damage sigma g is achieved by the known phenomenon of crack ramification.
A glass ceramic taught by the invention is preferably characterized by a composition in weight percent on an oxide basis of lithium oxide comprising three and zero percent to four percent, sodium monoxide comprising zero percent to one and zero percent, potassium monoxide comprising zero percent to six tenths percent, with the sum of sodium monoxide plus potassium monoxide comprising two tenths percent to one percent, magnesium oxide comprising zero percent to one and five tenths percent, calcium oxide comprising zero percent to five tenths percent, strontium oxide comprising zero percent to one percent, barium oxide comprising zero percent to two and five tenths percent, with the sum of calcium oxide plus strontium oxide plus barium oxide comprising two tenths percent to three percent, zinc oxide comprising one percent to two and two tenths percent, aluminium oxide comprising more than nineteen and eight tenths percent to twenty-three percent, silicon dioxide comprising sixty-six percent to seventy percent, titanium dioxide comprising two percent to three percent, zirconium dioxide comprising five tenths percent to two percent, and phosphoric oxide comprising zero percent to one percent.
A glass ceramic also taught by the invention is preferably characterized by a composition in weight percent on an oxide basis of: lithium oxide comprising three percent to four percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, sodium monoxide comprising zero percent to one percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, potassium monoxide comprising zero percent to six tenths percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, with the sum of sodium monoxide plus potassium monoxide comprising two tenths percent to one percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, magnesium oxide comprising zero percent to one and five tenths percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, calcium oxide comprising zero percent to five tenths percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, strontium oxide comprising zero percent to one percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, barium oxide comprising zero percent to two and five tenths percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, with the sum of calcium oxide plus strontium oxide plus barium oxide comprising two tenths percent to three percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, zinc oxide comprising one percent to two and two tenths percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, aluminum oxide comprising more than nineteen and eight tenths percent to twenty-three percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, silicon dioxide comprising sixty-six percent to seventy percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, titanium dioxide comprising two percent to three percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, zirconium dioxide comprising five tenths percent to two percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range, and phosphoric oxide comprising zero percent to one percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range.
For the realization of the structure of the translucent glass ceramic claimed by the invention, lithium oxide, zinc oxide, aluminum oxide and silicon dioxide are necessary in the indicated limits. These components are ingredients of the high quartz and keatite mixed crystals. The relatively narrow ranges are necessary so that the desired structure is formed. The aluminum oxide content should be greater than nineteen and eight tenths weight percent, because otherwise undesirably high silicon dioxide contents in the high quartz mixed crystals near the surface are favored. The aluminium oxide content is less than twenty-three weight percent because high aluminum oxide contents can lead to the undesirable devitrification of mullite when the molten glass is being molded. Magnesium oxide and phosphoric oxide can be incorporated as additional components. The addition of the alkalis sodium monoxide and potassium monoxide and the earth alkalis calcium oxide, strontium oxide and barium oxide improve the fusibility and the devitrification behavior of the glass during manufacture. The content levels are limited because these components essentially remain in the residual glass phase of the glass ceramic, and excessive levels undesirably increase the thermal expansion. The indicated minimum totals for the alkalis and earth alkalis are necessary so that the structure taught by the invention can be realized. The titanium dioxide content is between two weight percent and three weight percent, the zirconium dioxide content is between five tenths weight percent and two weight percent. Both are essential as nucleation agents. The translucent glass ceramic can be produced during the manufacturing process by the addition of conventional fining agents such as arsenic pentasulfide, antimony trioxide, tin dioxide, ceric oxide, sulfate and chloride compounds.
The translucent glass ceramic claimed by the invention can be manufactured in a variety of colors in accordance with the needs and desires of the market. If a high white value in the LAB System of L* greater than eighty-three is desired, the content of coloring impurities, in this case especially vanadium pentoxide, molybdenum trioxide, cobalt oxide and nickel oxide must be kept to extremely low levels. For example, vanadium pentoxide should be less than fifteen parts per million, molybdenum trioxide less than twenty parts per million, cobalt oxide less than ten parts per million, nickel oxide less than ten parts per million and the sum of the above mentioned color oxides should be less than thirty parts per million. On the other hand, if certain tints of the white color are desired, the conventional coloring components such as, for example, vanadium, chromium, manganese, cerium, iron, cobalt, copper, nickel, selenium and chlorine compounds can be used to achieve defined colors in the LAB system. To achieve a beige shade, for example, the addition of ceric oxide, manganese dioxide, iron oxide individually or in combination has proven successful.
For economic reasons it is advantageous if, from the same composition, in addition to the translucent glass ceramic, it is also possible to manufacture an opaque glass ceramic with light transmission less than five tenths percent at a specimen thickness of four millimeters. In this case, it is advantageous to perform the transformation into an opaque glass ceramic with keatite mixed crystal at elevated temperatures, because the resulting high quartz mixed crystals near the surface can thereby be decreased and the problem of the low quartz phase transition in high quartz mixed crystals that are high in silicon dioxide, which accompanies extended hold times, can be bypassed. An additional embodiment can be realized in the form of the opaque glass ceramics.
It is further economically advantageous if a transparent glass ceramic with high quartz mixed crystals as the predominant crystal phase and a transmission or, in other words, light transmissivity, of greater than eighty percent at a four millimeters specimen thickness can also be manufactured from the same composition from which the translucent glass ceramic is manufactured. This is possible if, after the crystallization of the high quartz mixed crystal, there is no temperature increase for the transformation into a translucent glass ceramic that contains keatite mixed crystals.
Preferably, a glass ceramic as claimed by the invention or a glass ceramic manufactured using the method taught by the invention is used in translucent or opaque form as a cooking surface or as cookware and in transparent form as fire protection glass, smokestack view windows, cookware or windows for combustion furnaces.
The white value and color in the LAB color system were measured using a measurement instrument against a black background.
The test of the ability of the glass ceramic plates claimed by the invention to withstand large temperature differences is performed on the basis of a typical stress situation when the glass ceramic plate is used as a cooking surface. A section of the glass ceramic plate to be tested, large enough for the test (generally a square section having the dimensions two hundred and fifty millimeters by two hundred and fifty millimeters) was mounted horizontally. The underside of the glass ceramic plate was heated by means of a conventional circular radiant heater, of the type conventionally used on cooktops, whereby any temperature limiting devices that were present were deactivated. The upper side of the glass ceramic plate was shielded by a metal hood from external factors that might interfere with the measurement process. Also on the upper side, the temperature of the glass ceramic plate, which increased gradually during the heating process, was measured, and specifically at the hottest spot within the uneven surface temperature distribution which is typical of the heating systems used. The area of the plate edge to be tested in terms of its ability to withstand temperature differences thereby has an unheated minimum widthxe2x80x94measured as the minimum distance between the outer edge of the plate and the inner boundary of the lateral insulated edge of the radiant heaterxe2x80x94corresponding to the most critical positioning of the heaters on cooktops. During the heating process, the unheated outer area came under tangential tensile stresses. The temperature at the measurement position described above at which the glass ceramic plate breaks under the action of the tangential tensile stresses was used as a reference value for the ability of the glass ceramic to withstand temperature differences.
The impact strength was measured using the ball drop test based on DIN 52306. The measurement specimen was a square section (dimensions one hundred millimeter by one hundred millimeters) of the glass ceramic panel to be tested and was mounted in a test frame. A two hundred grams steel ball was allowed to fall on the center of the specimen. The height of the drop was increased in steps until the glass ceramic panel broke. On account of the statistical character of the impact strength, this test was performed on a series of approximately ten specimens and the average of the ball drop heights measured was determined.
The above-discussed embodiments of the present invention will be described further hereinbelow. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintain that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.